Olefin(s) are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). This process is referred to as, the oxygenate-to-olefin process. Typically, the preferred oxygenate for light olefin production is methanol. The process of converting methanol-to-olefin(s) is called the methanol-to-olefin(s) process.
There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. The most common process for producing methanol is a two-step process of converting natural gas to synthesis gas. Then, synthesis gas is converted to methanol.
Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Synthesis gas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Synthesis gas is then processed into methanol. Specifically, the components of synthesis gas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one process, methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol is then converted to olefin(s) in a methanol-to-olefin(s) process. The methanol-to-olefin(s) reaction is highly exothermic and has a large amount of water. Water comprises as much as one half of the total weight of the output stream of the reactor or effluent stream. Consequently, the water must be removed by condensation in a quench device to isolate the olefin product. A quench device cools the effluent stream to the condensation temperature of water.
The effluent stream of an oxygenate-to-olefin reactor also contains byproducts including oxygenate byproducts such as organic acids, aldehydes and/or ketones. Carbon dioxide is also a byproduct of the oxygenate to olefin reaction. Furthermore, depending upon operating conditions, unreacted methanol is likely to be present in the effluent of the oxygenate-to-olefin reaction.
U.S. Pat. Nos. 6,482,998 and 6,121,504 describe an oxygenate-to-olefin process that includes a quench tower for removal of water produced in the oxygenate-to-olefin reactor. Unreacted oxygenate feed (typically methanol) that is liquid under quenching conditions is removed from the quench tower as a heavy product fraction. The unreacted oxygenate feed is separated from water in the quench medium in a fractionation tower.
U.S. Pat. No. 6,403,854 describes a two stage solids wash and quench for use with the oxygenate conversion process where catalyst fines are removed from the effluent stream through a first quench stage. The bottoms of the quench include water, alcohols, ketones and neutralized organic acids that have a boiling point greater than water. The quench medium is a portion of the quench bottoms that is mixed with a neutralization stream and purified water stream. Therefore acids such as formic acid, acetic acid, butyric acid and propanoic acid can be neutralized. The neutralization material can be caustic, amines or ammonia.
Notwithstanding the improvements in technology relating to the removal of impurities from the effluent stream of an oxygenate-to-olefin reactor with a quench device, it would be desirable to have a process for quenching an oxygenate-to-olefin effluent stream that will remove carbon dioxide as well as organic acids, aldehydes and/or ketones in the quench bottoms stream. The present invention addresses these and other needs.